Large refrigerating systems, e.g. for supermarkets, typically have one single compressing unit with a plurality of compressors working in parallel to provide compressed refrigerant via a condenser to a plurality of refrigerated spaces. In the refrigerated space, the refrigerant is evaporated in an evaporator whereby the temperature of the ambience, i.e. the temperature of the secondary fluid, is decreased. To adjust the temperature in separate refrigerated spaces individually, each of the spaces has separate evaporators with adjustable inlet valves. Usually, the inlet valve is temperature controlled, i.e. the valve of a refrigerated space opens and closes based on the temperature of the secondary fluid. If liquid refrigerant by accident leaves the evaporator, the compressors can be severely damaged. For that reason, the above-mentioned valve is usually inserted serially with a thermostatic valve which changes the flow rate based on the superheat of the refrigerant at the outlet of the evaporator. The thermostatic valve thus ensures that the refrigerant which is released into the evaporator is completely evaporated when it leaves the evaporator.
After the evaporation, vapour of refrigerant from each of the refrigerated spaces is led to an intake of the compressing unit. At the intake, suction pressure generated by the evaporated refrigerant is measured by a pressure gauge. If the suction pressure is high, the evaporation temperature is also high, and the required cooling may not be available. On the contrary, if the suction pressure is low, the efficiency of the compressors is reduced. In a traditional system, the compressing capacity of the compressing unit, i.e. the specific amount of refrigerant which is compressed, is controlled based on the suction pressure. When the pressure reaches an upper level, the compressing capacity is increased by switching on additional compressors, and when the pressure reaches a lower level, the compressing capacity is decreased by switching out additional compressors.
In one specific implementation, the compressor capacity is controlled by a PID based structure using the actual suction pressure as feedback. The compressor capacity can be controlled by use of the following mathematical expression
                              CC          ⁡                      (            t            )                          =                              K            p                    ⁡                      (                                                            e                  ⁡                                      (                    t                    )                                                  +                                                      1                                          T                      i                                                        ⁢                                      ∫                                                                  e                        ⁡                                                  (                          t                          )                                                                    ⁢                                              ⅆ                        t                                                                                                        ,                        )                                              Equation        ⁢                                  ⁢        1            with the control errore(t)=suction pressure setpoint(t)−actual suction pressure(t)  Equation 2
The compressor capacity control is divided into two terms, a proportional term and an integral term. The proportional part, shown as the first part of Equation 1, reacts directly on the actual control error. The integral term, shown as the last term of Equation 1, reacts on the integral of the control error. Hence, the integral term is responsible for eliminating steady state errors, and the proportional part reacts on set-point changes and control errors caused by changes in cooling demands. The tuning values Kp and Ti can be used to tune the controller to the system dynamics.
In a refrigeration system with more evaporators operating in hysteresis mode, the cooling demand varies much when the flow of refrigerant to the evaporators is switched in and out by an evaporator valve. This can have the undesirable effect on the compressor capacity control, that it will start or stop a compressor each time the evaporators switch in or out, which causes an increased wear on the compressors.
One problem is that the known PI or PID based compressor capacity control systems are only able to react in a causal way which in practice means that a short positive peak in the cooling demand can cause a start of a compressor, shortly after followed by a stop of the same or of another compressor of the system. In such a situation, a preferred operation would have been to continue without the starting and stopping of the compressor, i.e. to ignore the brief changes of the cooling demand.
In a system with discrete capacity values, one further problem related to a PID based controller is that the lower compressor capacity value will produce a small negative control error. The negative error causes the integral part to start a compressor whereby the control error becomes slightly positive with a compressor stop as a result. The effect can be seen as a limit-cycle on the compressor capacity, even with constant cooling demand. A remedy to avoid the limit-cycle can be to introduce a dead-band where the integral part is only updated when the numerical control error is larger than a given value. However, the general problem, i.e. that the PID based structure can only react in a causal way, remains.
In addition to the above mentioned problem of frequent compressor start/stop cycles, control of refrigeration systems is complicated by relatively long time constants. As an example, it takes long time from an evaporator valve is actuated until the temperature in a corresponding refrigerated space is changing, or it takes long time from a cover is removed from a refrigeration display case until the demand for additional cooling capacity is observed. On the other hand, the time it takes from the compressor capacity is changed to the change has an effect on the pressure on the suction side of the compressing unit, is relatively short.
In a refrigeration system of the kind mentioned in the introduction, fluctuations, e.g. due to switching of the evaporator valves are expected. An increased cooling demand could be caused by these fluctuations or it could be caused by a more permanent change of the cooling demand. If an increase is caused by fluctuation, it would not be suitable to change the compressor capacity, whereas if the change is of a more permanent nature, the compressor capacity should be changed. A traditional system, e.g. a PI(D) based system is not capable of determining if a cooling demand is caused by fluctuation, and in some cases, a traditional system would therefore react on fluctuation by regulating the compressor capacity by switching a compressor on or off unnecessarily, whereby compressor wear increases.